He’s interested in him because he’s his lead to cracking the drug case he’s working on. Instead of taking down a few people he wants to take down the whole system. Mr fox is his golden ticket since he’s close to the action (moves and possibly sells the product) and inexperienced enough that he will make mistakes and lead him to exactly what he wants. Plus he seems to find his resistance thrilling or something (he’s very obviously a creep) and since Mr fox is carrying his child, unfortunately, he gets a free excuse to be around him. Unless we get more info I’ll assume he’s been exposed just by being around it all the time through his job as the delivery person. It’s like when people get cancer from second hand smoking, they might not have been the ones doing it but the particles spread and illness does not discriminate. Something strong enough to genetically change betas is something that most likely should be handled with proper precautions like respirators and gloves, maybe special suits too.
So how was Fox slowly exposed to the drug over time? It showed flashbacks of him getting his mail and stuff. So does that mean someone was exposing him when he was unaware? (Secretly drugging him) Why does it sound like the prosecutor is the one that did that to him? Is there a backstory we haven't been told yet? I mean, it does seem like he's oddly REALLY interested with him.